Zinc alloy plated steel material having excellent weldability and processed-part corrosion resistance

ABSTRACT

Zn alloy plated steel material having excellent weldability and processed-part corrosion resistance and a method for production of Zn alloy plated steel material are provided. In the Zn alloy plated steel material comprising base steel material and a Zn alloy plating layer, the Zn alloy plating layer includes, by wt %, Al: 0.1-5.0%, Mg: 0.1-5.0%, as well as a remainder of Zn and inevitable impurities. The Zn alloy plated steel material includes a lower interface layer and an upper interface layer between the base steel material and the Zn alloy plating layer, wherein the lower interface layer is formed on the base steel material and has a dense structure, and the upper interface layer is formed on the lower interface layer and has a network-type or island-type structure.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/539,654, filed on Jun. 23, 2017, which is the U.S. NationalPhase under 35 U.S.C. § 371 of International Patent Application No.PCT/KR2015/014264, filed on Dec. 24, 2015, which in turn claims thebenefit of Korean Patent Application Nos. 10-2014-0188048, filed on Dec.24, 2014, 10-2014-0190124, filed on Dec. 26, 2014 and 10-2015-0186014,filed on Dec. 24, 2015, the entire disclosures of which

Applications are incorporated by reference therein.

TECHNICAL FIELD

The present disclosure relates to a zinc (Zn) alloy plated steelmaterial having excellent weldability and deformed-part corrosionresistance and to a method of manufacturing the same.

BACKGROUND ART

Since zinc (Zn) plating, suppressing the corrosion of iron (Fe) usingcathodic protection, has excellent anti-corrosion performance andeconomic efficiency, Zn plating has commonly been used in manufacturinga steel material having high corrosion resistance. In detail, in thecase of hot dip galvanized steel materials forming a plating layer insuch a manner that a steel material is immersed in molten Zn, themanufacturing process thereof is simpler, and the prices of products arecheaper than an electrogalvanized steel material. Thus, demand for hotdip galvanized steel materials has increased in the automobile industry,the household appliance industry, the building materials industry, andother industries.

Hot dip galvanized steel materials plated with Zn may have thecharacteristics of sacrificial corrosion protection in which, when beingexposed to a corrosion environment, Zn having an oxidation reductionpotential lower than that of Fe is first corroded, so that the corrosionof steel materials is prevented. In addition, Zn in a plating layer maybe oxidized to generate minute corrosion products on the surface ofsteel materials and to block steel materials from an oxidizingatmosphere, thereby improving corrosion resistance.

However, due to increased air pollution caused by industrialdevelopment, an increase of corrosive environments and strictregulations regarding resource conservation and energy savings, there isa growing need for the development of steel materials having bettercorrosion resistance than Zn plated steel materials of the related art.

To this end, a large amount of research into technology to manufactureZn alloy plated steel materials having improved corrosion resistancethrough adding elements, such as aluminum (Al) and magnesium (Mg) to agalvanizing bath, has been conducted. A large amount of research into atechnology to manufacture Zn—Al—Mg-based Zn alloy plated steelmaterials, as a typical Zn alloy plated material, in which Mg is addedto Zn—Al plated materials has also been conducted.

However, such Zn—Al—Mg-based Zn alloy plated steel materials haveweaknesses as below.

First, when Zn—Al—Mg-based Zn alloy plated steel materials are welded,cracks caused by liquid metal embrittlement (LME) may easily occur,thereby degrading weldability thereof. In other words, in a case inwhich Zn alloy plated steel materials described above are welded,Zn—Al—Mg-based intermetallic compounds having a relatively low meltingpoint are dissolved and penetrate between grain boundaries of the basesteel.

Second, Zn—Al—Mg-based Zn alloy plated steel materials have a low levelof deformed-part corrosion resistance. In other words, such Zn alloyplated steel materials may include a large amount of Zn—Al—Mg-basedintermetallic compounds generated by thermodynamic interactions of Zn,Al, and Mg in a plating layer. Since such intermetallic compounds have arelatively high degree of hardness, cracks may be generated in a platinglayer during a bending process, thereby degrading deformed-partcorrosion resistance.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a zinc (Zn) alloy platedsteel material having excellent weldability and deformed-part corrosionresistance and a method of manufacturing the same.

Technical Solution

According to an aspect of the present disclosure, a zinc (Zn) alloyplated steel material comprises a base steel and a Zn alloy platinglayer, the Zn alloy plating layer including, by wt %, aluminum (Al):0.1% to 5.0%, magnesium (Mg): 0.1% to 5.0%, Zn as a residual component,and inevitable impurities. Between the base steel and the Zn alloyplating layer, a lower interface layer formed on the base steel andhaving a fine structure and an upper interface layer formed on the lowerinterface layer and having a network-type structure or an island-typestructure are provided.

According to another aspect of the present disclosure, a method ofmanufacturing a Zn alloy plated steel material comprises providing abase steel; surface activating the base steel; obtaining the Zn alloyplated steel material by immersing the base steel that has been surfaceactivated in a Zn alloy plating bath including, by wt %, Al: 0.1% to5.0%, Mg: 0.1% to 5.0%, Zn as a residual component, and inevitableimpurities and performing plating; and cooling the Zn alloy plated steelmaterial after gas wiping the Zn alloy plated steel material.

Advantageous Effects

According to an aspect of the present disclosure, a Zn alloy platedsteel material has significantly excellent weldability and deformed-partcorrosion resistance.

DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscope (SEM) image of an interfacelayer of a zinc (Zn) alloy plated steel sheet according to InventiveExample 1 of Exemplary Example 1.

FIG. 2 is an SEM image of an interface layer of a Zn alloy plated steelsheet according to Comparative Example 1 of Exemplary Example 1.

FIG. 3 is an SEM image of an interface layer of a Zn alloy plated steelsheet according to Specimen Number 1 of Exemplary Example 2.

FIG. 4 is an SEM image of an interface layer of a Zn alloy plated steelsheet according to Specimen Number 2 of Exemplary Example 2.

FIG. 5 is an SEM image of an interface layer of a Zn alloy plated steelsheet according to Specimen Number 3 of Exemplary Example 2.

FIG. 6 is an SEM image of an interface layer of a Zn alloy plated steelsheet according to Specimen Number 4 of Exemplary Example 2.

BEST MODE FOR INVENTION

Hereinafter, a zinc (Zn) alloy plated steel material having excellentweldability and deformed-part corrosion resistance according to anexemplary embodiment will be described in detail.

A Zn alloy plated steel material according to an exemplary embodimentincludes a base steel and a Zn alloy plating layer. In an exemplaryembodiment, a type of the base steel is not specifically limited, andthe base steel may be provided as a steel sheet or a steel wire rod. Inthe meantime, a Zn alloy plating layer may be formed on a single surfaceor opposing surfaces of the base steel.

In addition, in an exemplary embodiment, an alloy composition of thebase steel is not specifically limited. However, in a case in which thebase steel includes, a total of 0.1 wt % or more of one or more type ofsurface enrichment element selected from a group consisting of silicon(Si), manganese (Mn), and nickel (Ni), a portion of surface enrichmentelements in the base steel is dissolved (a total of 0.001 wt % or more)in an upper interface layer and a lower interface layer formed betweenthe base steel and a plating layer, thereby maximizing an effect of anexemplary embodiment.

The Zn alloy plating layer may include, by wt %, aluminum (Al): 0.1% to5.0%, Mg: 0.1% to 5.0%, Zn as a residual component, and inevitableimpurities.

Mg in the Zn alloy plating layer is an element playing a role inimproving corrosion resistance of a plated steel material. In a case inwhich an Mg content is significantly low, there may be a problem inwhich an effect of improving corrosion resistance is insignificant.Thus, a lower limit value of the Mg content in the Zn alloy platinglayer may be 0.1 wt %, in detail, 0.5 wt %, and more specifically, 0.8wt %. However, in a case in which the Mg content is significantly high,there is a problem in which dross of a plating bath caused by oxidationof Mg in the plating bath may occur. Thus, an upper limit value of theMg content in the Zn alloy plating layer may be 5.0 wt %, in detail, 3.0wt %, and more specifically, 2.0 wt %.

Al in the Zn alloy plating layer is an element playing a role insuppressing dross of an Mg oxide. In a case in which an Al content issignificantly low, an effect of preventing oxidation of Mg in theplating bath is insignificant. Thus, a lower limit value of the Alcontent in the Zn alloy plating layer may be 0.1 wt %, in detail, 0.5 wt%, and more specifically, 0.8 wt %. However, in a case in which the Alcontent is significantly high, there is a problem in which a temperatureof the plating bath should be raised. In a case in which the temperatureof the plating bath is relatively high, plating equipment may be eroded.Thus, an upper limit value of the Al content in the Zn alloy platinglayer may be 5.0 wt %, in detail, 3.0 wt %, and more specifically, 2.0wt %.

Between the base steel and the Zn alloy plating layer, the lowerinterface layer formed on the base steel and having a fine structure andthe upper interface layer formed on the lower interface layer and havinga network-type structure or an island-type structure may be provided.

A crack of liquid metal embrittlement (LME) considered to mainly be aproblem during spot welding of the Zn alloy plated steel material may beeffectively suppressed by forming an interface layer having a doublelayered structure, as described above. Even in the case in which, acrack is generated on a surface of the Zn alloy plating layer in abending process, bending workability may be improved by effectivelypreventing the base steel from being outwardly exposed.

According to an exemplary embodiment, an area percentage of the upperinterface layer, as compared with an area of the lower interface layer,may be in a range of 10% to 90%, in detail, in a range of 20% to 80%, inmore detail, in a range of 40% to 70%, and more specifically, in a rangeof 45% to 65%. In this case, an area percentage refers to a ratio of anarea of the upper interface layer to an area of the lower interfacelayer, when viewed from an upper portion of a steel material in athickness direction thereof, for example, in the case of a flat surface,without considering three dimensional bending or the like. In a case inwhich the area percentage of the upper interface layer is less than 10%,the area of the upper interface layer is significantly small, so thatweldability and deformed-part corrosion resistance of the Zn alloyplated steel material may be degraded. On the other hand, in a case inwhich the area percentage of the upper interface layer is more than 90%,a crack may be generated due to brittleness during a process.

In this case, whether the interface layer having the double layeredstructure, as described above, has been formed or not can be determinedusing a method below. In other words, since the interface layer havingthe double layered structure is present on an interface of the basesteel and the Zn alloy plating layer, there is a limitation inconfirming a structure thereof, or the like, in a case in which the Znalloy plating layer is not removed. Thus, after an entirety of the Znalloy plating layer was dissolved by dipping the Zn alloy plated steelmaterial in a chromic acid solution, chemically dissolving only the Znalloy plating layer on an upper portion of the upper interface layer for30 seconds, while the interface layer having the double layeredstructure was not damaged, a scanning electron microscope (SEM) image ofa residual interface layer, as described above, was taken, and it wasdetermined whether the interface layer having the double layeredstructure was formed or not by analyzing an image, in order to measure athickness of each interface layer. In this case, as an example toproduce the chromic acid solution, the chromic acid solution may beproduced in such a manner that one liter of distilled water is mixedwith 200 g of CrO3, 80 g of ZnSO4, and 50 g of HNO3. In the meantime, acomposition of each interface layer to be subsequently described may beanalyzed using energy dispersive spectroscopy (EDS), while the areapercentage of the upper interface layer may be measured using an imageanalyzer.

According to an exemplary embodiment, the upper interface layer and thelower interface layer include an iron (Fe)—Al-based alloy. TheFe—Al-based alloy may be provided as one or more type of alloy selectedfrom a group consisting of Fe₂Al₅, FeAl₃, and FeAl. In this case, whenthe upper interface layer and the lower interface layer include theFe—Al-based alloy, the Fe—Al-based alloy is included as a main component(about 80 wt % or more), and the case in which other effectivecomponents or inevitable impurities are included is not excluded.

According to an exemplary embodiment, the upper interface layer mayinclude, by wt %, Al: 15% to 80%, Fe: 20% to 85%, and Zn: 10% or less(including 0%), in detail, Al: 15% to 60%, Fe: 40% to 80%, and Zn: 10%or less (including 0%), and more specifically, Al: 20% to 40%, Fe: 60%to 80%, and Zn: 10% or less (including 0%).

In general, the Al content in an interface layer formed at an interfacebetween the Zn alloy plating layer and the base steel is about 10 wt %.However, in the case of the Zn alloy plated steel material according toan exemplary embodiment, the Al content contained in the upper interfacelayer is relatively high. In a case in which the Al content in the upperinterface layer is lower than 15%, an effect of reducing an LME crackmay be insignificant. On the other hand, in a case in which the Alcontent is higher than 80%, a crack may be generated due to brittlenessduring a process.

According to an exemplary embodiment, a thickness of the upper interfacelayer may be in a range of 50 nm to 1,000 nm, in detail, in a range of70 nm to 800 nm, in more detail, in a range of 75 nm to 450 nm, and morespecifically, in a range of 90 nm to 420 nm. In a case in which thethickness of the upper interface layer is less than 50 nm, the effect ofreducing the LME crack may be insignificant during welding. On the otherhand, in a case in which the thickness thereof is greater than 1,000 nm,an area of a crack may become greater during a process.

According to an exemplary embodiment, a thickness of the lower interfacelayer may be 500 nm or less (excluding 0 nm), in detail, 300 nm or less(excluding 0 nm), and more specifically, 100 nm or less (excluding 0nm). In a manner different from the upper interface layer, the lowerinterface layer should conformally cover a front surface of the basesteel. However, in a case in which the thickness of the lower interfacelayer is greater than 500 nm, the lower interface layer is unlikely tobe able to conformally cover a surface of the base steel. In themeantime, on condition that the lower interface layer conformally coversthe surface of the base steel, in general, when the thickness thereof isthinner, a degree of uniformity thereof is increased. Thus, a lowerlimit value thereof is not specifically limited.

The Zn alloy plated steel material according to an exemplary embodimentdescribed above may be manufactured using various methods, and amanufacturing method thereof is not specifically limited. However, theZn alloy plated steel material may be manufactured using a method belowas an implementation example.

Hereinafter, a method of manufacturing the Zn alloy plated steelmaterial having excellent weldability and deformed-part corrosionresistance according to another exemplary embodiment will be describedin detail.

Surface Activating

After a base steel is provided, the base steel is surface activated.Surface activating base steel is performed to easily form an Fe—Al-basedalloy layer having a double layered structure between the base steel anda Zn alloy plating layer.

According to an exemplary embodiment, a centerline arithmetical averageroughness (Ra) of a surface activated base steel may be in a range of0.8 μm to 1.2 μm, in detail, in a range of 0.9 μm to 1.15 μm, and morespecifically, in a range of 1.0 μm to 1.1 μm. In this case, thecenterline arithmetical average roughness (Ra) refers to an averageheight from a centerline (an arithmetical mean line of a profile) to acurve of a cross section.

In addition, according to an exemplary embodiment, a ten point medianheight (Rz) of the surface activated base steel may be in a range of 7.5μm to 15.5 μm. In this case, the ten point median height (RZ) refers toa distance between two parallel virtual lines passing through a thirdhighest point and a third lowest point, respectively, in a roughnessprofile taken in a reference length of a cut-off portion thereof andparallel with the centerline.

In addition, according to an exemplary embodiment, a maximum heightroughness (Rmax) of the surface activated base steel may be in a rangeof 8 μm to 16.5 μm. In this case, the maximum height roughness (Rmax)refers to a distance between two parallel virtual lines parallel withthe centerline and passing through a highest point and a lowest point ofa curve in the roughness profile taken in a reference length of acut-off portion thereof.

In a case in which surface roughnesses (Ra, Rz, and Rmax) of the basesteel is controlled to be within a range described above, a reactionbetween the base steel and a plating solution is relatively active,thereby easily forming an interface layer having a double layeredstructure.

In an exemplary embodiment, a method of surface activating the surfaceof the base steel is not specifically limited. However, in detail, aplasma treatment or an excimer laser treatment may be used. Duringplasma treatment or excimer laser treatment, a specific processingcondition is not specifically limited. Any device and/or condition thatmay activate the surface of the base steel to within a range describedabove may be applied.

However, as a desirable example to activate the surface of the basesteel, a method below may be used.

The surface activating the base steel may be performed using plasmatreatment on the condition of radio frequency (RF) power in a range of150 W to 200 W. In a case in which RF power is controlled to be within arange described above, an area percentage of an upper interface layer,as compared with an area of a lower interface layer, may be optimized,thereby securing excellent weldability and deformed-part corrosionresistance.

In addition, the surface activating the base steel may be performed inan inert gas atmosphere. In this case, the inert gas atmosphere may beprovided as either a nitrogen gas (N₂) atmosphere or an argon (Ar) gasatmosphere. As such, in a case in which the surface activating the basesteel is performed in the inert gas atmosphere, an oxide film present onthe surface of the base steel is removed, thereby improving reactivityof the plating solution and the base steel. Thus, the Fe—Al-based alloylayer having a double layered structure may be easily formed between thebase steel and the Zn alloy plating layer.

Forming Surface Oxide Layer

A surface oxide layer is formed on a surface of a base steel by heattreating the base steel. However, in the case of forming a surface oxidelayer, if the base steel includes, by wt %, a total of 0.1% or more ofone or more of type of element selected from a group consisting of Si,Mn, and Ni, surface enrichment of Si, Mn, and Ni may be carried out,thereby sufficiently dissolving Si, Mn, and Ni in an interface layerformed in a subsequent process. The forming a surface oxide layer is notessential.

In the meantime, in a case in which the forming a surface oxide layer isperformed before obtaining a Zn alloy plated steel material, a processorder is not specifically limited. For example, after surface activatingthe base steel, the surface oxide layer may be formed in a surfaceactivated base steel. Alternatively, after the surface oxide layer isformed, the base steel in which the surface oxide layer has been formedmay be surface activated.

According to an exemplary embodiment, during the heat treating the basesteel, a heat treatment temperature may be in a range of 700° C. to 900°C., and in detail, in a range of 750° C. to 850° C. In a case in whichthe heat treatment temperature is lower than 700° C., an effect thereofmaybe insufficient. On the other hand, in a case in which thetemperature thereof is higher than 900° C., process efficiency may bereduced.

Obtaining Zn Alloy Plated Steel Material

A surface activated base steel or a surface activated base steel inwhich a surface oxide layer has been formed is immersed in a Zn alloyplating bath including, by wt %, Al: 0.1% to 5.0%, Mg: 0.1% to 5.0%, Znas a residual component, and inevitable impurities, and plating isperformed, thereby obtaining a Zn alloy plated steel material.

In this case, a temperature of a plating bath of the related art may beapplied to a temperature of the Zn alloy plating bath. In general, in acase in which an Al content, among components in the plating bath, isincreased, a melting point is increased. Thus, internal equipment of theplating bath is eroded, thereby causing a reduction in a lifespan of adevice and a defect on a surface of a Zn alloy plated steel material dueto an increase in dross of an Fe alloy in the plating bath. However,since the Al content is controlled to be in a range of 0.5 wt % to 3.0wt %, a relatively low level, it is unnecessary to set the temperatureof the plating bath to be relatively high, and the temperature of theplating bath of the related art maybe applied. In detail, thetemperature thereof may be in a range of 430° C. to 480° C.

Subsequently, a coating weight is controlled using a gas wipingtreatment of the Zn alloy plated steel material. The gas wipingtreatment is to control the coating weight, and a method thereof is notspecifically limited. Air or N₂ may be used as a gas in this case, andit is more desirable to use N₂ therebetween. In a case in which air isused, Mg may be first oxidized on a surface of a plating layer, therebycausing a defect on the surface of the plating layer.

Subsequently, the Zn alloy plated steel material, the coating weight ofwhich has been controlled, is cooled. In an exemplary embodiment, duringcooling described above, a cooling rate and a cooling end temperatureare not specifically limited and may be set based on a cooling conditionof the related art. In the meantime, during cooling described above, acooling method is not specifically limited. In detail, cooling may beperformed by using an air jet cooler, N₂ wiping, by spraying water mist,or the like.

Hereinafter, an exemplary embodiment will be described in more detailusing an exemplary example. However, an exemplary embodiment below isintended to describe the present disclosure in more detail throughillustration thereof, but not limit the scope of rights of the presentdisclosure, because the scope of rights thereof is determined by thecontents written in the appended claims and reasonably inferredtherefrom.

INDUSTRIAL APPLICABILITY Exemplary Example 1

After, as a specimen for plating, a low carbon cold rolled steel sheethaving a thickness of 0.8 mm, a width of 100 mm, and a length of 200 mmwas provided, a surface thereof was surface activated using a plasmatreatment. In this case, Ra, Rz, and Rmax of the surface activated basesteel are illustrated in Table 1 below. Subsequently, the surfaceactivated base steel was immersed in a Zn alloy plating bath having acomposition in Table 1 below, thereby manufacturing a Zn alloy platedsteel material. Subsequently, a coating weight was controlled to be 70g/m² per side by gas wiping the Zn alloy plated steel material, and theZn alloy plated steel material was cooled to a room temperature (about25° C.) at an average cooling rate of 10° C./sec.

Subsequently, a composition, a thickness, and an area percentage of aninterface layer of each manufactured Zn alloy plated steel material weremeasured, and results thereof are illustrated in Table 1 below. Ameasuring method is the same as a case described above.

Subsequently, weldability and deformed-part corrosion resistance of eachmanufactured Zn alloy plated steel material were assessed, and resultsthereof are illustrated in Table 2 below.

Weldability was assessed using a method below.

A welding current of 7 kA was applied using a copper (Cu)-chrome (Cr)electrode having a tip diameter of 6 mm, and welding was performed onthe condition of electrode force of 2.1 kN, a welding time of 11 cycles(in this case, 1 cycle refers to 1/60 second and is the same,hereinafter) and a holding time of 11 cycles. In each exemplary example,a total of five specimens were produced. Lengths of an entirety of LMEcracks occurring in five specimens were measured, and an average lengthof LME cracks and a maximum length of LME cracks were obtained. As aresult, in a case in which the average length of LME cracks was 20 μm orless, a specimen was assessed to be “GO”. In a case in which the averagelength of LME cracks was greater than 20 μm, a specimen was assessed tobe “NG”. In a case in which the maximum length of LME cracks was 100 μmor less, a specimen was assessed to be “GO”. In a case in which themaximum length of LME cracks was greater than 100 μm, a specimen wasassessed to be “NG”.

Deformed-part corrosion resistance was assessed using a method below.

After a bending process (0T bending) of each Zn alloy plated steelmaterial at 180° C., each bending-processed Zn alloy plated steelmaterial was charged into a salt spray testing instrument, and a redrust occurrence time was measured based on international standard (ASTMB117-11). In this case, 5% salt water (at a temperature of 35° C. and apH of 6.8) was used, and 2 ml/80 cm² of water was sprayed per hour. In acase in which the red rust occurrence time was 500 hours or more, the Znalloy plated steel material was assessed to be “GO”. Ina case in whichthe red rust occurrence time was shorter than 500 hours, the Zn alloyplated steel material was assessed to be “NG”.

TABLE 1 Compo- sition Upper Interface Lower Sur- of Layer Interface facePlating Com- Percen- Layer Rough- Bath po- Thick- tage Compo- Thick- Re-ness (wt %) sition ness (area sition ness marks (μm) Al Mg (wt %) (nm)%) (wt %) (nm) Inven- Ra: 1 1 Al: 830 25 Al: 80 tive 1.04 20.10 9.8Exam- Rz: Fe: Fe: ple 1 8.57 77.63 88.8 Rmax: Zn: Zn: 10.5 1.11 1.35Com- Ra: 1 1 — — — Al: 50 par- 0.56 10.3 ative Rz: Fe: Exam- 9.26 88.65ple 1 Rmax: Zn: 13.7 1.01 Com- Ra: 1.6 1.6 — — — Al: 60 par- 1.57 12.5ative Rz: Fe: Exam- 15.2 86.34 ple 2 Rmax: Zn: 12.8 1.15

TABLE 2 Deformed-part Corrosion Resistance Weldability Red Rust AverageMaximum Occurrence Length of Length of Time Remarks LME Cracks (μm) LMECracks (μm) (h) Inventive 18 GO 94 GO 500 GO Example 1 Compar- 30 NG 179NG 300 NG ative Example 1 Compar- 34 NG 125 NG 350 NG ative Example 2

With reference to Tables 1 and 2, it can be confirmed that, in the caseof Inventive Example 1 satisfying an entirety of conditions of anexemplary embodiment, an average length of LME cracks is 20 μm or less,a maximum length of LME cracks is 100 μm or less, and weldability isexcellent. In addition, a red rust occurrence time is 500 hours or more,and deformed-part corrosion resistance is significantly excellent. Inthe meantime, it can be confirmed that, in the case of ComparativeExamples 1 and 2, weldability and deformed-part corrosion resistance wasdegraded, since an interface layer having a double layered structure wasnot formed.

In the meantime, FIG. 1 is an SEM image of an interface layer of a Znalloy plated steel sheet according to Inventive Example 1 of ExemplaryExample 1. FIG. 2 is an SEM image of an interface layer of a Zn alloyplated steel sheet according to Comparative Example 1 of ExemplaryExample 1.

Exemplary Example 2

In order to assess a change in an area percentage of an upper interfacelayer depending on a condition of plasma treatment and weldability anddeformed-part corrosion resistance of a Zn alloy plated steel materialdue to the change, the Zn alloy plated steel material was manufacturedwhile other conditions were the same as those of Exemplary Example 1,and only a composition of a plating bath (Al of 1.4 wt %, Mg of 1.4 wt%, and Zn as a residual component) and a condition of plasma treatmentwere different. The condition of plasma treatment in each example isillustrated in Table 3 below.

Subsequently, a composition, a thickness, and an area percentage of aninterface layer of each manufactured Zn alloy plated steel material weremeasured, and results thereof are illustrated in Table 3 below. Ameasuring method is the same as a case described above.

Subsequently, weldability and deformed-part corrosion resistance of eachmanufactured Zn alloy plated steel material were assessed, and resultsthereof are illustrated in Table 4 below. An assessment method is thesame as a case described above.

TABLE 3 Condition of Upper Interface Lower Plasma Layer InterfaceTreatment Percen- Layer Speci- RF Compo- Thick- tage Compo- Thick- menPower Atmos- sition ness (area sition ness No. (W) phere (wt %) (nm) %)(wt %) (nm) 1 50 Air Al: 16 25 36 Al: 11.5 60 Fe: 81.5 Fe: 82.9 Zn: 2.5Zn: 5.6 2 100 N₂ Al: 25 55 24 Al: 18.4 55 Fe: 71.9 Fe: 78 Zn: 3.1 Zn:3.6 3 150 N₂ Al: 25.3 150 48 Al: 26.4 60 Fe: 72.2 Fe: 72.1 Zn: 2.5 Zn:1.5 4 200 Ar Al: 46.2 350 61 Al: 16.4 55 Fe: 50.2 Fe: 81.8 Zn: 3.6 Zn:1.8 5 250 Ar Al: 79 460 72 Al: 9.8 55 Fe: 15.2 Fe: 86.7 Zn: 5.8 Zn: 3.5

TABLE 4 Deformed-part Weldability Corrosion Average Maximum ResistanceLength of Length of Red Rust Specimen LME Cracks LME Cracks OccurrenceNo. (μm) (μm) Time (h) 1 28.6 102.5 500 2 18.8 96.7 550 3 11 68 800 49.4 59 900 5 8.5 57 600

With reference to Tables 3 and 4, it can be confirmed that, in the caseof specimens 3 and 4 in which an area percentage of an upper interfacelayer was controlled to be within a range of 40% to 70%, weldability anddeformed-part corrosion resistance was significantly excellent ascompared with other specimens.

FIG. 3 is an SEM image of an interface layer of a Zn alloy plated steelsheet according to Specimen Number 1 of Exemplary Example 2. FIG. 4 isan SEM image of an interface layer of a Zn alloy plated steel sheetaccording to Specimen Number 2 of Exemplary Example 2. FIG. 5 is an SEMimage of an interface layer of a Zn alloy plated steel sheet accordingto Specimen Number 3 of Exemplary Example 2. FIG. 6 is an SEM image ofan interface layer of a Zn alloy plated steel sheet according toSpecimen Number 4 of Exemplary Example 2.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

The invention claimed is:
 1. A zinc (Zn) alloy plated steel material,comprising: a base steel and a Zn alloy plating layer, the Zn alloyplating layer including, by wt %, aluminum (Al): 0.1% to 5.0%, magnesium(Mg): 0.1% to 5.0%, Zn as a residual component, and inevitableimpurities, wherein between the base steel and the Zn alloy platinglayer, a lower interface layer formed on the base steel and having afine structure and an upper interface layer formed on the lowerinterface layer and having a network-type structure or an island-typestructure are provided, wherein a maximum length of LME cracks at a weldzone of the Zn alloy plated steel material is 100 μm or less, where theweld zone is formed by a welding current of 7 kA was applied using acopper (Cu)-chrome (Cr) electrode having a tip diameter of 6 mm, andwelding was performed on the condition of electrode force of 2.1 kN, awelding time of 11 cycles and a holding time of 11 cycles, wherein 1cycle is 1/60 seconds.
 2. The Zn alloy plated steel material of claim 1,wherein the upper interface layer and the lower interface layer comprisean Fe—Al-based alloy, and the Fe—Al-based alloy is provided as one ormore type of alloy selected from a group consisting of Fe₂Al₅, FeAl₃ andFeAl.
 3. The Zn alloy plated steel material of claim 1, wherein an areapercentage of the upper interface layer, as compared with an area of thelower interface layer, is in a range of 10% to 90%.
 4. The Zn alloyplated steel material of claim 1, wherein an area percentage of theupper interface layer, as compared with an area of the lower interfacelayer, is in a range of 40% to 70%.
 5. The Zn alloy plated steelmaterial of claim 1, wherein the upper interface layer includes, by wt%, Al: 15% to 80%, Fe: 20% to 85%, and Zn: 10% or less (including 0%).6. The Zn alloy plated steel material of claim 1, wherein a thickness ofthe upper interface layer is in a range of 50 nm to 1,000 nm.
 7. The Znalloy plated steel material of claim 1, wherein a thickness of the upperinterface layer is in a range of 74 nm to 450 nm.
 8. The Zn alloy platedsteel material of claim 1, wherein a thickness of the lower interfacelayer is 500 nm or less (excluding 0 nm).
 9. The Zn alloy plated steelmaterial of claim 1, wherein the base steel comprises, by wt %, a totalof 0.1% or more of one or more elements selected from a group consistingof silicon (Si), manganese (Mn), and nickel (Ni), and the upperinterface layer and the lower interface layer comprise a total of 0.001%or more of one or more elements selected from a group consisting of Si,Mn, and Ni.
 10. The Zn alloy plated steel material of claim 1, wherein acenterline arithmetical average roughness (Ra) of the base steel is 0.8μm to 1.2 μm.
 11. The Zn alloy plated steel material of claim 1, whereina ten point median height (Rz) of the base steel is 7.5 μm˜15.5 μm. 12.The Zn alloy plated steel material of claim 1, wherein a maximum heightroughness (Rmax) of the base steel is 8 μm˜16.5 μm.
 13. The Zn alloyplated steel material of claim 1, wherein an average length of LMEcracks at a weld zone of the Zn alloy plated steel material is 20 μm orless, where the weld zone is formed by a welding current of 7 kA wasapplied using a copper (Cu)-chrome (Cr) electrode having a tip diameterof 6 mm, and welding was performed on the condition of electrode forceof 2.1 kN, a welding time of 11 cycles and a holding time of 11 cycles,wherein 1 cycle is 1/60 seconds.
 14. The Zn alloy plated steel materialof claim 1, wherein a red rust occurrence time of the Zn alloy platedsteel material is 500 hours, where the red rust occurrence time of theZn alloy plated steel material is measured based on ASTM B117-11, aftera bending process (OT bending) of each Zn alloy plated steel material.